A prior art linear encoder includes a scale fixed to a fixed body, and a head fixed to a mobile object that moves relatively to the fixed body to read the positional information of the head with respect to the scale by using the head and detect the position of the mobile object. For example, an optical linear encoder includes a head having a light-emitting device and a light-receiving device, and a scale having a grid scale to reflect light from the light-emitting device by the grid scale of the scale, receive reflected light by the light-receiving device and detect the position of the head, on the basis of a change in the quantity of light of the reflected light. Moreover, for example, a magnetic linear encoder includes a head having a magnetoresistive device, and a scale having a plurality of magnets alternately arranged so that mutually adjoining magnets have mutually reverse polarities to measure a change in the magnetic field corresponding to the scale provided respectively in correspondence with the north pole and the south pole of each magnet and detect the position of the head.
In general, these linear encoders need to have a gap between the head and the scale set to about 1 mm or less. When the gap is set to several centimeters or more in an optical encoder, the light irradiation point grows dim or the quantity of light of the reflected light becomes small, and this leads to decrease in the signal-to-noise ratio of the signal. Accordingly, there is a such problem that the position measurement error becomes large, and the measurement becomes impossible in the worst case. Further, there is such a problem that the positioning becomes impossible under the influence of dust and dirt and turbulence light. Moreover, there is a such problem that a strong magnet needs to be used in order to measure changes in the magnetic field when the gap is set to be equal to or larger than several centimeters in the magnetic encoder, and this leads to an increase in the cost of the scale. Moreover, there is such a problem that measurement becomes impossible under the influence of magnetic materials existing around the magnetic encoder.
There has been a demand for highly accurately detecting the train position in order to perform train operation control with high accuracy in the field of railroad, and a demand for highly accurately detecting the cage position in order to perform cage control with high accuracy in the field of elevator. When a linear encoder is used for railroad train position detection, the gap between the head and the scale needs to be 10 cm to 40 cm. Moreover, when a linear encoder is used for elevator cage position detection, the gap between the head and the scale needs to be several centimeters.
However, in the prior art optical or magnetic linear encoder, there are many problems upon widely setting the gap between the head and the scale to several centimeters or more as described above, and it is difficult to apply these linear encoders directly to the railroad train position detection and the elevator cage position detection.
In order to solve these problems, the Patent Document 1 discloses a detector apparatus, which includes a head that is fixed to a mobile object and transmits and receives radio waves, and a scale that changes the intensity of reflection of radio waves fixed to a fixed body to detect the position by detecting a change in the reflection intensity of radio waves in the head. Since the radio waves are used in the detector apparatus of the Patent Document 1, the position of the mobile object can be detected without receiving the influences of dust and dirt, turbulence light and magnetic materials existing around the detector apparatus. Moreover, since the change in the reflection intensity of radio waves only needs to be able to be detected, the gap between the head and the scale can be set widely by comparison to the prior art.